1. Field of the Invention
The invention relates to a propeller shaft used as a driving force transmitting shaft portion of a vehicle.
2. Description of the Related Art
Japanese Patent Application Publication No. 2007-177955 (JP-A-2007-177955) and Japanese Patent Application Publication No. 2009-107415 (JP-A-2009-107415), for example, describe a conventional propeller shaft. This propeller shaft is arranged between a transmission that is connected to a power unit, and a differential gear that is connected to wheels. The propeller shaft transmits output from the transmission to the differential gear. The position of the transmission relative to the differential gear changes according to impacts from the road surface and the operating state of the vehicle and the like. Therefore, in order to absorb relative displacement of these devices in the longitudinal direction of the vehicle in particular, the propeller shaft is formed of a first shaft having male splines and a second shaft having female splines. The male splines and the female splines are formed so as to be slidable relative to each other in the axial direction (i.e., the longitudinal direction of the vehicle).
With this kind of propeller shaft, the slide resistance between the male splines and the female splines is large. A phenomenon known as stick-slip may occur due to relative movement between the transmission and the differential when the vehicle is accelerating or decelerating or the like. Stick-slip is a phenomenon in which the male splines of the first shaft and the female splines of the second shaft intermittently slide relative to each other in the axial direction.
Therefore, JP-A-2007-177955 describes coating the sliding surfaces of the male splines and the female splines with a solid lubrication film of diamond-like carbon (hereinafter also referred to as “DLC”) or the like in order to reduce the occurrence of stick-slip.
This DLC film is described in Japanese Patent Application Publication No, 2009-35584 (JP-A-2009-35584), for example. The DLC film is formed on the surface of a substrate using any of a variety of known film forming methods such as a plasma Chemical Vapor Deposition (CVD) method or a Physical Vapor Deposition (PVD) method such as a vacuum deposition method. Among these film forming methods, a direct-current plasma CVD method is generally well used because the film forming conditions are easy to control.